1. Field of the Invention
The present invention relates to a mechanically processing arrangement for a workpiece having an optically sensed position detecting pattern or patterns formed thereon, and more particularly, relates to an arrangement for accurately positioning a workpiece at a predetermined position suitable for a mechanical processing of the workpiece by the utilization of a position detecting pattern or patterns formed on the workpiece, and a position aligning system having an opto-electric position detector unit including a line sensor, and mechanically processing the same by means of a machine tool, such as a severing tool, a scoring tool, a grooving tool, or a boring tool, with a high accuracy.
2. Description of the Related Art
When a workpiece, such as a semiconductor wafer, is separated into a plurality of individual pieces, such as rectangular semiconductor chips, a severing apparatus having a cutting tool and a worktable for mounting a workpiece, which is capable of moving both in the X- and Y-axes lying in a plane and rotating about an axis perpendicular to the plane, is employed. The worktable on which a workpiece is mounted, is fed in the X- and Y-axes and is rotated about the rotating axis so that a desired severing position of the workpiece is accurately brought into a predetermined position suitable for severance by the cutting tool. That is, the severing apparatus must be provided with an accurate position aligning unit.
A typical alignment system is disclosed in, for example, GB No. 2 139 348 A, in which a part of the surface of a workpiece, i.e., a surface area of the workpiece including a position to be severed, is taken as a two dimensional image by a camera to detect the position to be severed. The taken image is output and subjected to a differential computation so as to detect position information of a street formed in the severed position with respect to both the X- and Y-axes. Thus, the detected position to be severed and having the street is aligned with a cutting tool.
Another position alignment apparatus is disclosed in Japanese Examined Patent Publication No. 59-43820, granted to the Hitachi Mfg. Company of Japan. The position alignment apparatus is adapted for carrying out a position alignment between a semiconductor wafer and a masking, and is provided for establishing a relative position alignment between a first objective article, such as a semiconductor wafer, having a first aligning pattern and a second objective article, such as a masking, having a second aligning pattern. The apparatus is characterized by comprising an optical system for focussing an image formed by overlapping the first and second aligning patterns, an image sensor for obtaining image signals of the image in the X- and Y-axis directions, and an aligning means for detecting a displacement between the first and second objective articles in both the X- and Y-axes on the basis of the image signals in the X- and Y-axes delivered from the image sensor, and causing a relative movement of the first and second objective articles in the X- and Y-axes direction until the displacement is eliminated.
Operation of the above-mentioned two conventional position alignment system is based upon the technique of opto-electrically converting an objective article or articles into a two dimensional image or images thereof, and then processing the electric signals of the image or images to achieve the position alignment. Therefore, a long time is required to complete the processing of the electric signals of the two dimensional images. Further, the use of the two dimensional image of an objective article brings about a problem such that there is a limited resolution in the detection of the two dimensional image of the objective article. Accordingly, the accuracy of the position detection of the objective article is insufficient. As a result, when the conventional position alignment system is accommodated in a mechanical processing machine, such as a wafer severing machine, the accuracy of the mechanical processing of the objective article, such such as a semiconductor wafer, is insufficient. Also, it takes a long time to carry out the mechanical processing of the objective article, and as a result, during the mechanical processing operation, a rise in temperature occurs, due to friction between a processing tool and the objective article. This temperature raise will cause an error in the position alignment of the objective article.